The present invention relates to a vehicle, particularly a tractor, which is switchable between a two wheel drive mode and a four wheel drive mode.
A four wheel drive vehicle wherein front wheels and rear wheels are driven simultaneously is well known. It is desirable that the front wheels and rear wheels are driven at an equal vehicle speed when the vehicle travels straight in the four wheel drive mode. However, when the vehicle makes a turn as shown in FIG. 14 of the accompanying drawings, a turning radius R2 of front wheels FW is greater than a turning radius R1 of rear wheels RW. Therefore, the vehicle has a great turning radius unless the vehicle speed of the front wheels is increased over the vehicle speed of the rear wheels. The vehicle speed of the front wheels herein means a vehicle speed realized by the rotation of the front wheels, and the vehicle speed of the rear wheels means a vehicle speed realized by the rotation of the rear wheels.
Thus, it is necessary for the four wheel drive vehicle to have different vehicle speeds of the front and rear wheels for straight running and turning. However, it is extremely difficult to change the vehicle speeds of the front and rear wheels to be optimal speeds now for straight running, now for turning, since a four wheel drive mechanism for distributing the drive of an engine to the front wheels and rear wheels normally comprises transmission gears.
A proposal has been made to set the vehicle speed of the front wheels to be faster than that of the rear wheels, thereby causing either the front wheels or the rear wheels (the front wheels in the case of a tractor) to slip relative to the ground when the vehicle runs straight, and permitting the vehicle to make a turn within a small radius by the high speed rotation of the front wheels.
In another proposal made heretofore, the front wheels and the rear wheels have the vehicle speed ratio of about 1:1 and a one-way clutch is mounted between front wheel driving shafts for transmitting the drive only from the engine to the front wheels and not vice versa. According to this construction, the four wheel drive mode is selected for straight running whereas, when the vehicle makes a turn, the one-way clutch is operated to break the drive transmission and place the front wheels in free rotation thereby to permit the vehicle to turn within a small radius.
A further proposal already made comprises a switch mechanism for selectively providing a two wheel drive mode and a four wheel drive mode. According to this construction, the vehicle is driven only by the rear wheels when running straight and by the four wheels when making a turn or when the rear wheels slip.
The construction in which the front wheels are set to a higher vehicle speed than the rear wheels has the problem of wearing tires through the slips occurring between the front wheels and the ground. The construction including a one-way clutch has the problem of rendering engine brakes unavailable. The construction for selectively providing the two wheel drive mode and four wheel drive mode has the problems of involving troublesome operations and of poor vehicle maneuverability.